Minor: Consistency of `self.invalid` in InchCoord/Location

InchCoord and InchLocation previously used `_unset` or `self.isInvalid` (weird) to check the object's state in getters.  Now they both consistently use `self.invalid`.

Also added a comment to InchCoord's `_doubleValue` property to make it clear that it should be NAN whenever the InchCoord is invalid (as opposed to specifying a custom accessor for doubleValue that has to do an extra check at time of get).